Scanning Transmission Electron Microscopy Characterization Research Articles

Insights into radiation damage from atomic resolution scanning transmission electron microscopy imaging of mono-layer CuPcCl16 films on graphene

Atomically resolved images of monolayer organic crystals have only been obtained with scanning probe methods so far. On the one hand, they are usually prepared on surfaces of bulk materials, which are not accessible by (scanning) transmission electron microscopy. On the other hand, the critical electron dose of a monolayer organic crystal is orders of magnitudes lower than the one for bulk crystals, making (scanning) transmission electron microscopy characterization very challenging. In this work we present an atomically resolved study on the dynamics of a monolayer CuPcCl16 crystal under the electron beam as well as an image of the undamaged molecules obtained by low-dose electron microscopy. The results show the dynamics and the radiation damage mechanisms in the 2D layer of this material, complementing what has been found for bulk crystals in earlier studies. Furthermore, being able to image the undamaged molecular crystal allows the characterization of new composites consisting of 2D materials and organic molecules.

Strain Engineering to Enhance the Electrooxidation Performance of Atomic-Layer Pt on Intermetallic Pt3Ga.

Strain engineering has been a powerful strategy to finely tune the catalytic properties of materials. We report a tensile-strained two-to-three atomic-layer Pt on intermetallic Pt3Ga (AL-Pt/Pt3Ga) as an active electrocatalyst for the methanol oxidation reaction (MOR). Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy characterization showed that the AL-Pt possessed a 3.2% tensile strain along the [001] direction while having a negligible strain along the [100]/[010] direction. For MOR, this tensile-strained AL-Pt electrocatalyst showed obviously higher specific activity (7.195 mA cm-2) and mass activity (1.094 mA/μgPt) than those of its unstrained counterpart and commercial Pt/C catalysts. Density functional theory calculations demonstrated that the tensile-strained surface was more energetically favorable for MOR than the unstrained one, and the stronger binding of OH* on stretched AL-Pt enabled the easier removal of CO*.

Room temperature NO2 gas sensor based on porous Co3O4 slices/reduced graphene oxide hybrid

The porous flake-like Co3O4 and Co3O4/graphene hybrids with different graphene contents were prepared through a facile two-step method. The field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) characterizations proved the tight adherence of graphene sheets to the surface of porous Co3O4 slices. Gas sensors based on the as-synthesized products were then fabricated to explore their potential applications. The results indicated that the optimal hybrid exhibited a response of 26.8% to 5 ppm of NO2 at room temperature (RT), which was 2.27 times higher than that of undoped Co3O4 (100 °C). The hybrid sensor also showed fast response, excellent selectivity, long-term stability and extremely low detection limit toward NO2 at RT. The significantly enhanced sensing properties to NO2 could be attributed to larger specific surface area, more chemisorbed oxygen species and the coupling effect between Co3O4 and graphene in the hybrid. As we all know, as a typical p-type semiconductor, the gas-sensing properties of Co3O4 toward oxidizing gases including NO2 was rarely reported. We believe that our work could pave the way for the future design of Co3O4-based gas sensors for oxidizing gases detection at low or even room temperature.

Ball-in-ball hierarchical design of P2-type layered oxide as high performance Na-ion battery cathodes

Hierarchical microstructures assembled with nanostructures can effectively enhance the electrochemical performances of electrode materials for rechargeable Li/Na-ion batteries. Due to the large ionic radius of sodium, this kind of design and synthesis is more important for Na-battery cathodes, which represent a cost-effective alternative for energy storage applications. Here ball-in-ball hierarchical microspheres assembled with micron shaped flakes of the P2-type Na0.7Ni0.18Mn0.64Co0.18O2 have been designed and synthesized via hydrothermal reaction followed by stepwise calcination process. As a cathode material of the Na-ion battery, this novel and uniform hierarchical structure exhibits very high specific capacity and superior rate performance with discharge capacities of 208 mAh g−1 at 0.05 C and 46 mAh g−1 at 10 C, and an excellent cycling stability with about 78% capacity retention at 1 C after 50 cycles. These performances are better than any layered oxide cathode material for reversible Na-ion batteries reported in the literature, prepared via the conventional solid-state method. Combining ex-situ x-ray absorption spectroscopy characterization, x-ray diffraction, field emission scanning electron microscopy, scanning transmission electron microscopy and electrochemical characterization, we demonstrate that the superior performances can be attributed to the unique ball-in-ball hierarchical structure assembled with micron shaped flakes. This configuration could effectively reduce the path of Na ion diffusion, increases the contact area between electrodes and electrolyte and buffers the volume changes during the Na ion insertion/extraction processes.

Structural, morphological and optical properties of sheet like WO3 nanostructure and its catalytic activities for photoelectrochemical water splitting

Herein, sheet like WO3 nanostructures were synthesized by facile hydrothermal method in the presence and absence of urea. The phase formation and crystallinity of the synthesized samples were investigated by using powder X-ray diffraction technique, which revealed that the synthesized nanostructures in the presence of urea showed the high crystalline quality than the nanostructures synthesized in the absence of urea. Morphology and size of the synthesized nanostructures were identified from field-emission scanning electron microscope and transmission electron microscopy characterizations, which showed the sheet like morphology. The optical band gap was calculated as 2.57 and 3.03 eV for WO3 nanostructures synthesized in the presence and absence of urea respectively. Moreover, the photocurrent density was measured to study the photoelectrochemical water splitting performance of the synthesized nanostructures. The photocurrent density of 0.800 mA cm−1 at 1.2 V s. Ag/AgCl and 0.011 mA cm−1 at 1.2 V s. Ag/AgCl for WO3 nanostructures synthesized in the presence and absence of urea respectively. The higher photocurrent density of the WO3 nanostructures synthesized in the presence of urea was due to the improved charge transport property.

Carrier Localization in GaN/AlN Quantum Dots As Revealed by Three-Dimensional Multimicroscopy.

The localization of carrier states in GaN/AlN self-assembled quantum dots (QDs) is studied by correlative multimicroscopy relying on microphotoluminescence, electron tomography, and atom probe tomography (APT). Optically active field emission tip specimens were prepared by focused ion beam from an epitaxial film containing a stack of quantum dot layers and analyzed with different techniques applied subsequently on the same tip. The transition energies of single QDs were calculated in the framework of a 6-bands k.p model on the basis of APT and scanning transmission electron microscopy characterization showing that a good agreement between experimental and calculated energies can be obtained, overcoming the limitations of both techniques. The results indicate that holes effectively localize at interface fluctuations at the bottom of the QD, decreasing the extent of the wave function and the band-to-band transition energy. They also represent an important step toward the correlation of the three-dimensional atomic scale structural information with the optical properties of single light emitters based on quantum confinement.

Exploring the Potential of Different-Sized Supported Subnanometer Pt Clusters as Catalysts for Wet Chemical Applications

The use of physicochemical preparation techniques of metal clusters in the ultrahigh vacuum (UHV) allows for high control of cluster nuclearity and size distribution for fundamental studies in catalysis. Surprisingly, the potential of these systems as catalysts for organic chemistry transformations in solution has not been explored. To this end, single Pt atoms and Pt clusters with two narrow size distributions were prepared in the UHV and applied for the hydrogenation of p-chloronitrobenzene to p-chloroaniline in ethanol. Following the observation of very high catalytic turnovers (approaching the million molecules of p-nitroaniline formed per Pt cluster) and of size-dependent activity, this work addresses fundamental questions with respect to the suitability of these systems as heterogeneous catalysts for the conversion of solution-phase reagents. For this purpose, we employ scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS) characterization before and after react...

An advanced CoSe embedded within porous carbon polyhedra hybrid for high performance lithium-ion and sodium-ion batteries

A novel composite containing CoSe and porous carbon polyhedra (PCP), denoted as CoSe@PCP, was successfully synthesized using Co-based zeolitic imidazolate framework (ZIF-67) as precursor through a two-step method, including carbonization of ZIF-67 and subsequent selenization. The field-emission scanning electron microscopy and transmission electron microscopy characterizations confirm that CoSe nanoparticles are uniformly dispersed in PCP. When the CoSe@PCP was used as anode material for lithium-ion batteries, it exhibits superior performance with a high reversible capacity of 675mAhg−1 at 200mAg−1 after 100 cycles and 708.2mAhg−1 at 1Ag−1 after 500 cycles as well as excellent cycling stability. Additionally, the CoSe@PCP also demonstrates excellent performance as anode material for sodium-ion batteries. A reversible capacity of 341mAhg−1 can be obtained over 100 cycles at 100mAg−1 with high cycling stability. The excellent battery performance of CoSe@PCP should be attributed to the synergistic effect of nanostructured CoSe and PCP derived from ZIF-67, in which the nanostructured CoSe possesses high reactivity towards lithium and sodium ions and the PCP can provide a continuous conductive matrix to facilitate the charge transfer and an effective buffering to mitigate the structure variation of CoSe during cycling.

Incorporation of Zinc Oxide into Carbon nanotube/Graphite nanofiber as high performance supercapacitor electrode

An efficient and simple hydrothermal method has been developed to prepare Carbon nanotube/Graphite nanofiber/Zinc Oxide (CNT/GNF/ZnO) ternary composites which are employed as supercapacitor materials. The electrochemical measurements reveal that the introduction of ZnO into CNT/GNF can enhance the specific capacitance value up to 306 Fg−1 at 10 mVs−1, with 99.4% capacity retention. Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) characterizations reveal that the combination of nanometer-sized CNT/GNF and random distribution of impregnated ZnO can form a highly conductive network. The improved supercapacitance property of CNT/GNF/ZnO can be ascribed to the synergistic effect between double layer capacitance of CNT/GNF and the pseudocapacitance of ZnO.

Solid-state fabrication of Al/CNTs nanocomposites

In the present work, synthesis of Al-based composites reinforced with carbon nanotubes has been carried out using solid–state manufacturing routes exclusively to consolidate Al–1100/CNTs laminates. Preliminary results regarding consolidation of Al/CNTs nanocomposites in which pressure and temperature are combined in a single step are presented. Preliminary Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) characterization allow us to conclude that CNTs were effectively set on the Al matrix.

Contact resistance and stability study for Au, Ti, Hf and Ni contacts on thin-film Mg2Si

We present a detailed study of post-deposition annealing effects on contact resistance of Au, Ti, Hf and Ni electrodes on Mg2Si thin films. Thin-film Mg2Si and metal contacts were deposited using magnetron sputtering. Various post-annealing temperatures were studied to determine the thermal stability of each contact metal. The specific contact resistivity (SCR) was determined using the Cross Bridge Kelvin Resistor (CBKR) method. Ni contacts exhibit the best thermal stability, maintaining stability up to 400 °C, with a SCR of approximately 10−2 Ω-cm2 after annealing. The increased SCR after high temperature annealing is correlated with the formation of a Mg-Si-Ni mixture identified by cross-sectional scanning transmission electron microscopy (STEM) characterization, X-ray diffraction characterization (XRD) and other elemental analyses. The formation of this Mg-Si-Ni mixture is attributed to Ni diffusion and its reaction with the Mg2Si film.

Origin of the Normal and Inverse Hysteresis Behavior during CO Oxidation over Pt/Al2O3

The hysteresis in the CO oxidation profile over Pt/Al2O3 catalysts has been intensively studied, but its origin is still controversial. In this work, the influence of the Pt particle size and size distribution was systematically investigated. By the application of conventional and advanced preparation methods, such as flame spray pyrolysis and supercritical fluid reactive deposition, a series of catalysts containing homogeneous distributions of Pt particles were obtained. An optimal Pt particle size of 2–3 nm was identified for the CO oxidation light-off on Pt/Al2O3 catalysts. The CO oxidation results show a clear correlation between the Pt nanoparticle size and the ignition/extinction profile, including the switch of the hysteresis loop. On the basis of high-angle annular dark-field scanning transmission electron microscopy characterization combined with in situ diffuse reflectance infrared Fourier transform spectroscopy and operando X-ray absorption spectroscopy measurements, the appearance of inverse h...

Calculation and validation of a grain boundary complexion diagram for Bi-doped Ni

A grain boundary (GB) “phase” (complexion) diagram is computed via a lattice type statistical thermodynamic model for the average general GBs in Bi-doped Ni. The predictions are calibrated with previously-reported density functional theory calculations and further validated by experiments, including both new and old aberration-corrected scanning transmission electron microscopy characterization results as well as prior Auger electron spectroscopy measurements. This work supports a major scientific goal of developing GB complexion diagrams as an extension to bulk phase diagrams and a useful materials science tool.

Chemical intercalation of solvated sodium ions in graphite

Due to its similar electrochemistry to that of the lithium (Li)-ion battery but the high abundance of sodium (Na) in the earth, Na-ion battery is expected to be a promising alternative of the former, especially in stationary energy storage applications. Although it has been well-known that the Na-ions can be co-intercalated in graphite in some ether-based electrolytes, a number of important questions related to the Na-ion intercalation remain unclear. In order to better understand the sodium intercalation in graphite, Na-ions were intercalated in highly oriented pyrolytic graphite (HOPG) by immersing the latter in a sodium-biphenyl-monoglyme (Na-BP-DME) solution. The staging behavior that often occurs during Li-ion intercalation in graphite was observed in the Na-intercalated HOPG, on the basis of X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) characterization. The chemical formula of the DME-Na co-intercalated HOPG was determined to be Na(DME)2C26 by a combination of Fourier-transformed infrared (FTIR) and Raman spectroscopy, thermogravimetry and the above structural characterizing techniques. The negligible impact of co-intercalation on the structural integrity of HOPG demonstrates that HOPG can be a promising anode material of the Na-ion batteries. The lower coordination number between the Na+ ions and DME and the interaction between DME and the graphite layers are believed to be responsible for the structural stability of the co-intercalated graphite. Finally the chemical stability of the ternary graphite intercalation compounds in air was evaluated.

High-Yield Synthesis of Hollow Octahedral Silver Nanocages with Controllable Pack Density and Their High-Performance Sers Application.

Nanoparticle-assembled octahedral Ag nanocages with sharp edges have been successfully synthesized through a Cu2 O-based template-assisted strategy. In the reaction system, Ag nanoparticles can be self-assembled on the surface of Cu2 O octahedrons, which is accomplished by the reduction of Ag+ by NaBH4 in the presence of sodium citrate as a capping agent. The hollow octahedral Ag nanocages are obtained after removing the inner Cu2 O cores with acetic acid. According to the scanning electron microscopy (SEM) and transmission electron microscopy characterization, the Ag nanocages are weaved by small nanoparticles, the rough surfaces are bestrewed with pores and sharp edges. It is found that the pack density of Ag nanoparticles strongly affects the surface enhanced Raman scattering (SERS) activities. The as-prepared 1.05-Ag cages with optimal pack density have suitable interparticle distance and suitable size of pores, which significantly enhance SERS signals. The SERS signals of rhodamine 6G (R6G) molecules can be detected at an ultralow concentration of 10-14 m when 1.05-Ag cages are used as substrates. In addition to sensitivity, 1.05-Ag cages also exhibit good reproducibility. It is expected that the ultrahigh sensitivity will endow the Ag nanocages to become a promising candidate as high-performance SERS-based chemical sensor.

The influence of Cu addition and pre-straining on the natural aging and bake hardening response of Al-Mg-Si alloys

The effect of pre-straining on the natural aging and bake hardening of Al-Mg-Si alloys with different additions of Cu were investigated by Vickers microhardness measurements, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterization. It is revealed that introducing pre-straining can suppress the natural aging and increase the bake hardening response (BHR) for both low and high Cu addition alloys. A pre-strain of 2% is found to be the optimum process. Addition of Cu can increase the effect of pre-straining in suppressing the natural aging and improving the BHR after paint baking. The slow diffusion rate of Cu atoms in the Al matrix is most probably the main reason for the significant suppression of natural aging in the high content Cu alloy after pre-straining. During paint baking, the dislocations introduced promote the preferential precipitation of the Q′ phase and produce a disordered structure within the precipitates. Thus designing new pre-straining processes should fully consider the influences of Cu addition to the alloys.

Synthesis and Electrochemical Evaluation of Carbon Supported Pt-Co Bimetallic Catalysts Prepared by Electroless Deposition and Modified Charge Enhanced Dry Impregnation

Carbon-supported bimetallic Pt-Co cathode catalysts have been previously identified as higher activity alternatives to conventional Pt/C catalysts for fuel cells. In this work, a series of Pt-Co/C catalysts were synthesized using electroless deposition (ED) of Pt on a Co/C catalyst prepared by modified charge enhanced dry impregnation. X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) characterization of the base catalyst showed highly dispersed particles. A basic ED bath containing PtCl62− as the Pt precursor, dimethylamine borane as reducing agent, and ethylenediamine as stabilizing agent successfully targeted deposition of Pt on Co particles. Simultaneous action of galvanic displacement and ED resulted in Pt-Co alloy formation observed in XRD and energy dispersive X-ray spectroscopy (XEDS) mapping. In addition, fast deposition kinetics resulted in hollow shell Pt-Co alloy particles while particles with Pt-rich shell and Co-rich cores formed with controlled Pt deposition. Electrochemical evaluation of the Pt-Co/C catalysts showed lower active surface but much higher mass and surface activities for oxygen reduction reaction compared to a commercial Pt/C fuel cell catalyst.

Effects of high temperature pre-straining on natural aging and bake hardening response of Al–Mg–Si alloys

The influences of high temperature pre-straining (HT-PS) on the natural aging and bake hardening of Al–Mg–Si alloys were investigated by Vickers microhardness measurements, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterization. The results show that pre-straining at 170 °C immediately after quenching can effectively resolve the rather high T4 temper hardness caused by the conventional room temperature (RT) pre-straining treatment, and give a better bake hardening response (BHR) after paint-bake cycle. HT-PS 7% at 170 °C for 10 min is chosen as the optimum process as it provides lower T4 temper hardness and better BHR. The simultaneous introduction of dislocations and Cluster (2) can significantly suppress the natural aging and promote the precipitation of β″ phase, and reduce the effects of deformation hardening by dynamic recovery.

Electrochemical performance of aluminum niobium oxide as anode for lithium-ion batteries

AlNbO4, as lithium-ion batteries (LIBs) anode, has a high theoretical capacity of 291.5 mAh·g−1. Here, AlNbO4 anode materials were synthesized through a simple solid-state method. The structure, morphology and electrochemical performances of AlNbO4 anode were systematically investigated. The results show that AlNbO4 is monoclinic with C2/m space group. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations reveal the AlNbO4 particles with the size of 100 nm–2 μm. As a lithium-ion batteries anode, AlNbO4 delivers a high reversible capacity and good rate capability. The discharge capacity is as high as 151.0 mAh·g−1 after 50 charge and discharge cycles at 0.1C corresponding to capacity retention of 90.7 %. When the current density increases to 5.0C, AlNbO4 anode displays reversible discharge capacity of 73.6 mAh·g−1 at the 50th cycle.

Pt-Frame@Ni quasi Core–Shell Concave Octahedral PtNi3 Bimetallic Nanocrystals for Electrocatalytic Methanol Oxidation and Hydrogen Evolution

PtNi3 bimetallic concave octahedrons with a majority of platinum atoms deposited on the frames were synthesized in ethylene glycol solution. The high angle annular dark field scanning transmission electron microscopy (HAAD-STEM) characterizations and energy-dispersive X-ray spectroscopy (EDS) analysis reveal that the Pt frames have a thickness of less than 2 nm, which surround a nickel core thus forming a quasi core–shell concave octahedral nanoparticle (NP). The element-specific anisotropic growth followed by the nanoscale phase segregation and subsequent oxidation of Ni riched facets are responsible for the formation of the concave nanostructure. The PtNi3 quasi core–shell concave octahedrons exhibit substantially enhanced electrocatalytic properties toward methanol oxidation and hydrogen evolution reaction compared with that of the commercial Pt/C, suggesting that the Ni riched Pt–Ni NPs can be used as a potential candidate for methanol oxidation reaction (MOR) or hydrogen evolution reaction (HER) cata...